FIELD OF THE INVENTION
This invention relates to a method for producing two-phase, high strength, hot rolled steel sheet having excellent workability. Coild steel strips as well as cut steel sheets are collectively referred to herein as "sheet". "Low yield ratio" used herein means that ##EQU1## and the term "two-phase" or "two-phase structure" used herein means a steel structure which is composed essentially of the ferrite phase and the rapidly cooled transformation phase (containing mainly martensite with some amount of bainite and residual austenite).
In recent years, two-phase steel sheets with a low yield ratio have been increasingly adopted as press-forming steel materials in the automobile industry because of their excellent strength-ductility relationship, and in the field of hot rolling art for production of such two-phase steel sheets a method has been proposed (for example, Japanese Patent Publication No. Sho 55-49135, Japanese Laid-Open Patent Application No. Sho 56-29624, etc.), which comprises a low-temperature finishing and an extremely low temperature (lower than about 300.degree. C.) cooling. Therefore, it may be said the technical art in this field has been almost established.
However, in the prior art, it is generally necessary to add not less than 1.3% Mn to the steel, or if the manganese content is as low as 1.1% to 1.3%, for example, it is necessary to add at least 1% Si to the steel in order to obtain steel sheets having tensile strength of about 60 kg/mm.sup.2 so that the production cost is inevitably increased or some problems are unavoidable with respect to the descaling of the resultant hot rolled sheets or the paint adhesion of sheets in their ultimate applications.